Microbiology and Immunology

Hyphae WORLD OF MICROBIOLOGY AND IMMUNOLOGY

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glove can be an ideal breeding ground for bacteria.

Furthermore, the act of handwashing is to be done for a spec-

ified period of time and with vigorous rubbing together of

the hands and fingers. This is because the removal of

microorganisms is accomplished not only by the presence of

the soap but also by the friction of the opposing skin surfaces

rubbing together.

Other hygienic practices in a laboratory include wiping

the lab bench with a disinfectant compound before and after use

and keeping the work area orderly and free of debris. Protective

clothing can also be worn to minimize the spread of microor-

ganisms. Such clothing includes hair nets, disposal boots and

gloves, and lab coats. These items are worn in the vicinity of the

work bench or other areas where microorganisms are expected,

but are removed when exiting such locations.

Mechanical aids to hygiene exist. For example, many

labs contain a fume hood, in which airflow is directed inward.

Such laminar flow hoods do not allow the contaminated air

inside the hood to move outward into the laboratory. Another

standard feature of a microbiological laboratory is a small

flame source. The flame is used to sterilize the lip of test tubes

and vials before and after opening the containers, and to heat-

sterilize the metal inoculating loops used to transfer microor-

ganisms from one place to another.

While necessary for the protection of patients and to

prevent contaminationin the laboratory, the use of hygienic

substances can have adverse effects. In the late 1980s the so-

called “hygiene hypothesis” proposed that the increased use of

disinfectants, particularly in the home, had decrease the expo-

sure of people to substances that stimulated their immune sys-

tem, and so had rendered the immune system less capable of

dealing with environmental antigens. The result was proposed

to be an increase in allergies. Time has strengthened this

hypothesis to the point where the overuse of disinfectants has

become a legitimate concern.

In addition to the development of allergic reactions, the

inadequate or improper use of a hygienic compound can select

for organisms that are more capable of causing disease. For

example, certain disinfectants containing the compound tri-

closan have been shown to not only fail to kill the entire target

Escherichia colipopulation, but to actually stimulate the

development of resistance in those microbes that survive. In a

setting such as a kitchen, the results could be problematic.

See alsoAcne, microbial basis of; Antiseptics; Disinfection

and disinfectants; Transmission of pathogens

HHyphaeYPHAE

One of the biological characteristics that distinguish multicel-

lular fungifrom other organisms is their constitutional cells, or

hyphae (singular, hypha). Hyphae are nucleated cells in the

shape of thin tubes, externally enveloped in a rigid chitin-rich

cell wall and presenting an internal plasmatic membrane. They

contain cellular organelles such as mitochondria, Golgi appa-

ratus, ribosomes, endoplasmatic reticulum, which is also

found in other Eukaryotesas well as cytoplasmatic vesicles

bound to the plasmatic membrane. Hyphal cytoskeleton is

organized by microtubules. Hyphae are separated by walls

termed septae (singular, septum), usually bearing pores and

regulatory structures that prevent cellular leaking due to cell

disruption. For instance, the septum of Ascomycetes contains

the Woronin body, an oily structure that blocks the pore if cell

disruption occurs, whereas Basidomyceteshave a dolipore

septum, with the hyphae containing distinctly layered wall

structures and endoplasmatic reticulum next to the pore.

Hyphal growth and proliferation form structures similar to

fine branches, which form the myceliumor vegetative hyphal

network. However, Zygomycetes and Chytridiomycetes have

non-septate vegetative mycelium, except for the reproductive

structures.

As fungi grow, the older layers of hyphae gradually die

because growth occurs through the proliferation and branch-

ing of the apical cell of the mycelium (i.e., cell at the

mycelium tip). Growth takes place when two cytoplasmatic

vesicles bound to the internal membrane fuse at the apical

hypha, enlarging the hyphal tip because of the accumulation of

biomass, leading to septum formation and branching.

Branching is due to the growth of another apical cell inside the

sub apical region of the mother cell. The growth process so far

described is directed into new regions of the organic substrate,

from which the fungus is feeding, and is termed the extension

zone. The aerial part of the fungus, consisted of older hyphae

forming aerial mycelia, may develop and differentiate to form

structures bearing spores, and is termed the productive zone.

For most fungi, the haploid spore is the starting point

from which the haploid hypha will develop and form the

monokaryotic mycelium. The joining of two haploid mycelia

leads to diploidization of the apical cells, resulting in two cells

containing two separate nuclei, known as a dikaryotic

mycelium. These cells are capable of producing spores and in

Basidomycetes, are termed basidia (singular, basidium)

whereas in Ascomycetes, they are termed ascus. Usually, a

typical basidium produces four sexual spores and the ascus

can produce eight spores, although the amount of spores can

vary among the species of a given phylum. In the basidium,

for instance, the two nuclei are duplicated and then merged

when the cell is about to undergo meiosis twice, thus resulting

in the formation of four haploid spores. Some Ascomycetes as

well as a few Basidomycetes may also produce asexual

spores; and asexual reproduction is the way Zygomycetes and

Chytridiomycetes reproduce themselves.

See alsoFungal genetics; Fungi; Mycology; Parasites; Yeast

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